Above Ground Storage Tanks 2

[Inchtain]

Dear Engineers,

What does it mean when the label of a crude fixed doom roof tank indicates a partial stress relief?

Is it related to the welding of the clean out door? and how does it usually applied?

Thanks

 

[JStephen]

In the current standard, see Section 10.1.1 for the different markings to be used.
For an older tank, refer to the standard under which it was built.
But yes, this would usually be a flush manway, or flush shell nozzles, or shell nozzles/manways inserted and stress-relieved per 5.7.4.2 or 5.7.4.3, all of which are specified to be stress-relieved into a shell plate which is then welded into the tank.
For small tanks, it is sometimes feasible or desired to stress-relieve the whole tank, but this is uncommon.

 

[Inchtain]

Well! that's very clear.

However, out of my curiosity, how is it possible to stress relief the bottom reinforcement plate of the clean out door when only replacing it?

Thank you very much.

 

[IFRs]

API flush-typ cleanout assemblies are stress relieve (PWHT) prior to being installed into the tank.
The assembly includes the bottom reinforcing plate as well as a portion of shell plate.
See API 650 5.7.4.1:
5.7.4.1 All flush-type cleanout fittings and flush-type shell connections shall be thermally stress-relieved as an
assembly prior to installation in the tank shell or, alternatively, after installation into the tank shell if the entire tank is
stress-relieved. The stress relief shall be carried out within a temperature range of 600 °C to 650 °C (1100 °F to
1200 °F) (see 5.7.4.5 for quenched and tempered materials) for 1 hour per 25 mm (1 in.) of shell thickness. The
assembly shall include the bottom reinforcing plate (or annular plate) and the flange-to-neck weld.

 

[Inchtain]

Well! As in our case, we are going to only replace the bottom reinforcement plate (or annular plate) of the cleaning door.

Does we have to stress-relief the whole clean-out door assembly afterwards? or just PWHT of the new welding of the new annular plates to the new door reinforcement plate?

Or, in this case, is not required.

Thanks.

 

[IFRs]

In my opinion (and I tend to be conservative), replacing the thickened plate attached to the cleanout is probelmatic and will need PWHT, possibly by removing the whole assembly including the partial shell plate. In lieu of that, you might be able to use controlled deposition welding and more extensive NDE. An experienced tank engineer would be able to help you. Some of the decision involves the risk and consequence of a failure (leak). Do you actually need to remove and repolace the thickened plate or only the bottom plate lapped onto it?

 

[Inchtain]

Morning

Thank you for your swift reply.

Actually, the thickened plate has a metal loss of about 34%, while the butt welded annular plates with it has around 64% reduction on both sides.

All corrosion attacked these plates in the 40 cm area near the bottom lap-welded plates.

As long as we are replacing all the Tank floor, we guess that it is better to replace the door clean out reinforced bottom plate, as well.

Do you agree about that?

Best wishes,